Liquid sensing system

A system for detecting the presence or absence of a liquid in a volume comprising:

• sensor means having a pair of electrically conductive passive electrodes which are spaced apart by and extend from electrical insulating means for insertion into said volume,
• electronic oscillator means comprising an amplifier in amplifier means having an input and an output, one of said electrodes of said sensor means being electrically coupled to said amplifier input and the other to said amplifier output, said spaced apart electrically conductive electrodes of said sensor means and the space therebetween forming a sole feedback circuit signal path between said output and said input of said amplifier means, said feedback circuit in the absence of a liquid in the space between said two electrodes of said sensor means being open to provide substantially no feedback signal to said amplifier means so that said oscillator means will be non-oscillatory and said feedback circuit being closed to provide a positive feedback signal when liquid is present in said space due to a change in phase of the signal path between said two electrodes to cause said oscillator means to oscillate and thereby indicate the presence of a liquid in the space between said two electrodes;
• and
• the amplifier is connected to counter means, a frequency discriminator means and indicator means for producing an output control signal.